17β-Estradiol reverses shear-stress-mediated low density lipoprotein modifications

Within arterial bifurcations or branching points, oscillatory shear stress (OSS) induces oxidative stress mainly via the reduced nicotinamide adenine dinucleodtide phosphate (NADPH) oxidase system. It is unknown whether 17β-estradiol (E₂) can regulate OSS-mediated low-density lipoprotein (LDL) modifications. Bovine aortic endothelial cells were pretreated with E₂ at 5 nmol/L, followed by exposure to OSS (0 ± 3.0 dynes/cm² s and 60 cycles/min) in a flow system. E₂ decreased OSS-mediated NADPH oxidase mRNA expression, and E₂-mediated ^{{radical dot}}NO production was mitigated by the NO synthase inhibitor N(G)-nitro-l-argenine methyl ester. The rates of O₂ ^{-{radical dot}} production in response to OSS increased steadily as determined by superoxide-dismutase-inhibited ferricytochrome c reduction; whereas, pretreatment with E₂ decreased OSS-mediated O₂ ^{-{radical dot}} production (n = 4, p < 0.05). In the presence of native LDL (50 μg/mL), E₂ also significantly reversed OSS-mediated LDL oxidation as determined by high-performance liquid chromatography. In the presence of O₂ ^{-{radical dot}} donor, xanthine oxidase (XO), E₂ further reversed XO-induced LDL lipid peroxidation (n = 3, p < 0.001). Mass spectra acquired in the m/z 400-1800 range, revealed XO-mediated LDL protein nitration involving tyrosine 2535 in the α-2 domains, whereas pretreatment with E₂ reversed nitration, as supported by the changes in nitrotyrosine intensities. Thus, E₂ plays an indirect antioxidative role. In addition to upregulation of endothelial ^{{radical dot}}NO synthase and downregulation of Nox4 expression, E₂ influences LDL modifications via lipid peroxidation and protein nitration.